The present invention relates to communication antennas and, in particular, to a dielectric-coated, multi-flare angle, conical horn antenna for point-to-point communications, particularly home and commercial satellite.
Critical to the performance of any electromagnetic communication system are its transmitting and receiving antennas. The transmitting antenna is used to direct or focus radiated power in a desired direction toward a receiving antenna which is mounted to detect transmitted radiation with a minimum of noise from adjacent directions. The use of directional antennas exhibiting relatively high on-axis gain and minimal off-axis side lobes or other undesired signal characteristics enhances the ability to communicate point-to-point. A further desired attribute of such antennas is an ability to focus or amplify the free-field radiation without cross-polarization, since most communication channels use two linearly polarized signals whose electric fields are oriented at right angles to one another.
Due also to the high cost-per-unit-area of paraboloidal reflectors and interest in developing a television broadcast and/or data communication system using satellites in a geostationary orbit, not to mention satellite communications radar and radio astronomy, considerable interest exists to develop improved feed systems. Appreciating however that there is only one geostationary orbit, the equatorial orbit, it is anticipated that the demand for satellite positions in this orbit will continue to increase. To maximize utilization of this orbit, it will be necessary to space the satellites as closely as possible. This, in turn, will require satellite ground station antennas to radiate circularly polarized elliptical-shaped beams with high gain and directivity at low sidelobe levels. The low sidelobe levels avoid adjacent signal interference.
Moreover, if the cross-polarization radiation level is also kept low, then signals may be received on opposite polarizations, providing the facility of polarization diversity application, that therefore is, sending signals of different polarizations, such as will be necessary to meet various established communication standards. The requirement of antennas to meet this low cross-polarization condition is to have equal E-(Vertical) and H-(Horizontal) plane radiation patterns.
For satellite communications and other special applications, the directional beam may also require steering and thus an antenna with a variable beamwidth facility is preferred. Antennas for radio astronomy applications should have the combined features of low cross-polarization, suppressed sidelobes, a beam-shaping facility and wide bandwidth, in addition to high gain and greater directivity.
Current antennas, which are used to receive microwave and shorter wavelengths, frequently provide a relatively large reflective parabolic collector having broad-band gain characteristics. The collector is constructed to receive and focus the primary signal and side lobes, which are received due to the broad collector acceptance angles, at a separate receiving horn. That is, a co-axially mounted, rear-facing feedhorn capable of receiving broad beam widths, aligned with the signal axis and focal point of the collector, receives the focused signal and directs it to associated receiver electronics which appropriately convert and amplify the signal for its intended application.
The applicants have found however that over a number of bandwidths, centered on frequencies corresponding, for example to C and KU microwave bands, a forward-facing conical antenna having a small aperture, high gain and low side lobe characteristics can be used by itself, independent of a large surrounding collector. The entire antenna exhibits a size comparable to the feedhorn only of many current reflector antennas and in contrast thereto provides a much narrowed signal acceptance aperture.
In the latter regard, presently available home satellite systems predominantly operate at C-band frequencies and use down link antennas which measure ten to sixteen feet in diameter with relatively large flare angle feedhorns. Such antennas correspondingly require a relatively secure mounting system to prevent damage from wind and prevailing weather conditions.
Although the foregoing mounting problems are relatively easily overcome, the physical size of the antenna can present problems to users who reside in relatively dense population areas, especially in high rise buildings. That is, whereas the rural owner usually has available a larger unobstructed yard which permits relative freedom in positioning his/her antenna, the urban user may not have sufficient space to inconspicuously mount the antenna or may have to contend with neighboring structures which block reception. Furthermore, local ordinances or other legal or governmental restrictions may apply with respect to the mounting of such assemblies which may compound the user's problems.
Whereas the higher KU-band frequencies have been considered, as well as set aside for exclusive use with satellite communications, to date only a relatively few such satellites have been positioned in stationary earth orbit. An advantage of such antennas over C-band designs is that the antenna dish, using conventional constructions, can be constructed at diameters within the range of two to six feet, depending upon the transmission power levels of the satellite. Brody H., Big Hopes for Small Dishes, High Technology Business, pp. 41-45 (November, 1987). Such antennas, again, are typically constructed using conventional parabolic or other focusing collectors to collect and focus the received so-called "far field" signals onto a rear facing feedhorn, which typically is mounted to the antenna surface and aligned with the collector focal point. In contrast to C-band antennas which may weigh 200 pounds, collector type KU-band antennas commonly weigh only 100 pounds. In the latter regard, Applicants are also aware of an article discussing a flat array, KU-band antenna design. Long M., The Shape of Dishes to Come, Satellite Orbit, pp. 35-38 (October, 1987).
In further contrast to the foregoing, the present invention in one embodiment contemplates a KU-band antenna construction which provides for an antenna aperture in the range of only twelve to twenty-four inches and weighs less than five pounds. Numerous other constructions exhibit apertures less than ten inches and horn lengths less than fifteen inches. Such reduced dimensions are achieved through a uniquely arranged configuration of stages which will be described hereinafter. The construction is also such as to be compatible with a number of other frequency bands upon appropriate scaling.
To the extent Applicants are aware of antenna designs including features bearing some similarities of appearance to those of the subject invention, Applicant is aware of U.S. Pat. Nos. 2,761,141; 3,518,686; 3,917,773; and 3,866,234. Such references generally disclose variously shaped dielectric antenna lenses.
Applicants are also aware of U.S. Pat. Nos. 2,801,413; 3,055,004; 4,246,584; and 4,460,901 wherein the use of dielectric structures in association with horn antennas are shown.
Relative to multi-flared feedhorn antenna designs, Applicants are also aware of U.S. Pat. Nos. 2,591,486; 3,898,669; 4,141,015; and 4,442,437. Although disclosing stepped discontinuities within the antenna horn and although U.S. Pat. No. 3,898,669 discloses a multi-flared rectangular horn antenna, none of the noted references discloses the presently claimed combination of features for producing an antenna adaptable to a variety of frequencies, most particularly KU and C-band, and/or an antenna of the reduced dimensions and weight as exhibited by the antenna of the present invention. Such constructions, moreover, are intended for use as rear-facing feedhorns in combination with a large diameter, adjacent collector and not as stand-alone, forward-facing, far-field antennas.